Method for preparing barium-titanate based powder

ABSTRACT

The present invention relates to a method for preparing barium titanate based powder. More particularly, the present invention provides a method for preparing barium titanate powder comprising the following steps of precipitation of barium titanyl oxalate (BaTiO(C 2 O 4 ) 2 .4H 2 O) with spraying a mixture of an aqueous barium chloride (BaCl 2 .2H 2 O) and titanium tetrachloride (TiCl 4 ) to an aqueous solution of oxalic acid, via a nozzle; wet pulverization by using a beads mill after adding an additive such as an amine; dry; pyrolysis; and re-pulverization.

FIELD OF INVENTION

The present invention relates to a method for preparing barium titanatebased powder. More particularly, the present invention provides a methodfor preparing barium titanate powder comprising the following steps of:precipitating barium titanyl oxalate (BaTiO(C₂O₄)₂.4H₂O) by spraying anaqueous mixture of barium chloride (BaCl₂.2H₂O) and titaniumtetrachloride (TiCl₄) to an aqueous solution of oxalic acid, via anozzle; wet pulverization by using a beads mill after adding an additivesuch as an amine; drying; pyrolysis; and re-pulverization.

BACKGROUND OF THE INVENTION

The barium titanate powder has been widely utilized to producemulti-layer ceramic chip capacitors (MLCC), positive temperaturecoefficient thermistors, resistors, and the like. It is well-known thatbarium titanate powder can be manufactured via solid state reaction ofbarium carbonate (BaCO₃) and titanium dioxide (TiO₂) at hightemperature. As the trend in MLCC (multi-layer ceramic chip capacitor)continues towards further and further miniaturization with largecapacity, calcination at a low temperature, high frequency, andvolumetric efficiency, the demand for not only finer and more uniformbarium titanate powders has increased tremendously, but the need forpurity and distribution has also escalated as well. Thus, various liquidstate reaction methods such as hydrothermal method, co-precipitation(oxalate) method, and alkoxide method have been developed to producebarium titanate powders satisfying these characteristics.

The oxalate method is well discussed by W. S. Clabaugh et al. in Journalof Research of the National Bureau of Standards, Vol. 56(5),289–291(1956) to produce barium titanate by precipitating barium titanyloxalate with addition of a mixture solution containing Ba and Ti ions toan oxalic acid. However, this method has several drawbacks: (i) it isdifficult to control particle size and stoichiometric mole ratio of Bato Ti; (ii) hard aggregates between particles are formed during thepyrolysis, thus requiring strong pulverization to remove these hardaggregates; (iii) because extremely fine particles are enormouslyproduced during the strong pulverizing, it is hard to disperse thepowder for forming and abnormal grain growth occur during sinteringprocess. Thus, the barium titanate powders produced in this manner arenot adequate for the applications to multilayer ceramic capacitors.

Therefore, a hydrothermal method has been recently given attention tobecause of the trend of thinner and higher layered dielectric layer inMLCC. However, This method has disadvantages, like high manufacturingcost and complex process, due to use of autoclave, in spite of its highproduct quality. Therefore, there are increasing demands for developingsimpler methods for preparing barium titanate powders in low price to becompetitive in the market.

Inventors of the present invention have filed patent applications inSouth Korea (Korea Patent Application Nos. 2000-46125 and 2001-9066) forthe preparation of barium titanate powder which exhibit improved yieldwith shortened reaction time and optimized stoichiometry of barium totitanium. The method for preparing barium titanate powder disclosed inthe above Korea Patent applications comprises the steps of:precipitating barium titanyl oxalate (BaTiO(C₂O₄)₂

4H₂O) by spraying an aqueous mixture of barium chloride (BaCl₂

2H₂O) and titanium tetrachloride (TiCl₄) to an aqueous solution ofoxalic acid, via a nozzle in high speed and aging, filtering and washingthe same; pulverizing the obtained barium titanyl oxalate, drying, andpyrolizing to produce barium titanate (BaTiO₃) powder; andre-pulverizing the pre-pulverized barium titanate powder. In thepulverizing process, it can be performed by methods such as drypulverization using an atomizer and jet mill or wet pulverization usinga ball mill, planetary mill, and beads mill. In case it is needed toblend barium titanate (BaTiO₃) with other metal additives in thepulverizing step, wet pulverization is more preferable. Among wetpulverizing instruments, planetary mill or ball mill is suitable forlaboratory scale, while a beads mill is for industrial scale.

However, when the beads mill is employed for the wet pulverization ofbarium titanyl oxalate it causes the following problems:

(1) Impurities present in the solution occlude inside barium titanyloxalate particles during the precipitating step. Thus, impurities, suchas oxalic acid or chloride ion remain in barium titanyl oxalate preparedby co-precipitation (oxalate) method, no matter how many it is washed.Since this barium titanyl oxalate mixed with water has pH around 3, itmay cause corrosion of pre-mixer whose material is stainless steel.Further, it is difficult to control exothermic heat in case of usingacid-resistant material such as polyurethane. Therefore, it requires ause of expensive material like titanium as a pre-mixer. After the wetpulverization, the pH of the barium titanyl oxalate slurry becomes 2which is lower than that before the wet pulverization. Thus, it maydecrease material's durability of beads mill (inside part of millchamber) like zirconia and polyurethane. Furthermore it causes corrosionof a dryer.

(2) The presence of chloride ions in the barium titanyl oxalate willresult in the formation of a BaCl₂ liquid phase during sintering, whichis disclosed in Journal of Inorganic chemistry, vol. 9(11)2381˜89(1970). Thus, the calcined barium titanate aggregates more andexhibits poor dielectric characteristics.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to provide a processfor producing barium titanate powder economically by solving problemsassociated with the wet pulverization of barium titanyl oxalate in theprocess for preparing barium titanate disclosed in Korea Patent Nos.2000-46125 and 2001-9066.

BRIEF DESCRIPTION OF DRAWING

The above object of the present invention will become apparent from thefollowing description of the invention, when taken in conjunction withthe accompanying drawing, in which:

FIG. 1 represents SEM image of barium titanate powder produced by themethod of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a method for preparing barium titanatepowder comprising the steps of:

precipitating barium titanyl oxalate (BaTiO(C₂O₄)₂.4H₂O) by spraying anaqueous mixture of barium chloride (BaCl₂.2H₂O) and titaniumtetrachloride (TiCl₄) to an aqueous solution of oxalic acid, via anozzle;

wet pulverization by using a beads mill after adding an additive chosenfrom ammonia, amine, ammonium compound and amino acid;

pyrolizing to produce barium titanate powder; and

re-pulverization the obtained barium titanate powder.

The present invention will become apparent from the followingdescription of the invention, when the process for preparing bariumtitanate powder is described in more detail.

The first step is precipitation of barium titanyl oxalate with sprayingan aqueous mixture of barium chloride and titanium tetrachloride to anaqueous solution of oxalic acid via a nozzle in high speed, aging,filtration, and washing with water. The aqueous barium chloride solutionis prepared by dissolving barium chloride dihydrate (BaCl₂.2H₂O) inwater, and preferable barium chloride concentration is in the range offrom 0.2 to 2.0 mol/l. The aqueous titanium tetrachloride solution isprepared by diluting titanium tetrachloride solution, and preferabletitanium tetrachloride concentration is in the range of from 0.2 to 2.0mol/l. The mole ratio of the barium compound/titanium compound iscontrolled being in the range of from 1 to 1.5, more preferably 1 to 1.1when the aqueous solutions of barium chloride and titanium tetrachlorideare added each other. A concentration of the aqueous oxalic acidsolution is preferably in the range of from 0.2 to 5.0 mol/l and atemperature is maintained in the range of 20 to 100° C., more preferably50 to 90° C.

The prepared mixture of barium chloride and titanium tetrachloride isadded by spraying into an aqueous oxalic acid solution for 1 to 3 hoursthrough a nozzle. The nozzle used in the present invention may be asingle-fluid nozzle or double-fluid, preferably single-fluid nozzle as amatter of convenience.

The aging is performed for 0.5 to 2 hours and then the crude bariumtitanyl oxalate is washed with water till pH of the washer turns toneutral to produce the corresponding barium titanyl oxalate.

The second step is wet pulverization of the obtained barium titanyloxalate by means of a beads mill and drying. A solvent used for the wetpulverization is deionized water of which amount used is 1 to 10 partsby weight, relative to 1 part by weight of barium titanyl oxalate.During the process of the wet pulverization, a nitrogen-containingadditive is added to prevent from acidification of the mixture afterpulverization and decrease of dielectric characteristics of the powderdue to the presence of chloride ions within the barium titanyl oxalate.

Examples of the nitrogen-containing additive include ammonia, amines,ammonium compounds and amino acids. Particularly, use of an inorganicbase such as sodium hydroxide, potassium carbonate and sodiumbicarbonate is not preferable to remove chloride ions due to decrease ofdielectric characteristics of the powder such as dielectric constant andinsulation resistance. However, the nitrogen-containing additive used inthe present invention is preferable due to no containing of metalcations.

The amine used in the present invention is expressed by the followingformula (1),

wherein R₁, R₂, and R₃ represent individually hydrogen atom or C₁–C₅alkyl.

Examples of primary, secondary or tertiary amine include methylamine,dimethylamine, trimethylamine, diethylamine and triethylamine.

The ammonium compound used in the present invention is expressed by thefollowing formula (2),

wherein R₁, R₂, and R₃ represent individually hydrogen atom or C₁–C₅alkyl; A represents hydroxyl (n=1), nitrate (n=1), sulfate (n=2),phosphate (n=3), carbonate (n=2), bicarbonate (n=1), C₁–C₁₀ carboxyl(n=1), or C₂–C₁₀ dicarboxyl group (n=2).

Preferred examples of the ammonium compound include ammonium hydroxide,ammonium carbonate, ammonium acetate, ammonium phosphate, ammoniumoxalate, ammonium bicarbonate, and trimethylammonium hydroxide. Examplesof the amino acid include arginine and lysine.

The nitrogen-containing additive is used in the range of from 0.5 to 20mol %. If it is less than 0.5 mol %, it is undesirable in acidity andchloride content of the slurry. On the other hand, if it exceeds 20 mol%, it results in non-stoichiometric mole ratio of Ba to Ti due to lossof titanium component.

Further, a divalent or tetravalent metal may be arbitrarily incorporatedwith the nitrogen-containing additive and replaced for Ba or Tiposition. A divalent metal may be replaced for Ba and a tetravalentmetal may be for Ti. Examples of the divalent metal include Mg, Ca, Sr,and Pb and those of the tetravalent element include Zr, Hf, and Sn.These metals may be used in the form of oxides, carbonates, chlorides ornitrates. For example, an oxide, carbonate, chloride or nitrate of sucha replacement element is added to the barium titanyl oxalate to produceperovskite-type barium titanate based powder such asBa(Ti_(1-Z)Zr_(Z))O₃, (Ba_(1-x)Ca_(x)) (Ti_(1-z)Zr_(z))O₃,(Ba_(1-x-y)Ca_(x)Sr_(y)) (Ti_(1-z)Zr_(z))O₃, and the like.

The wet pulverized barium titanyl oxalate is dried and pyrolized to formbarium titanate in the presence of the additive. A rate for heatingduring the pyrolysis is preferably in the range of 0.5 to 10° C./min anda temperature is maintained at 700 to 1200° C.

The last step is re-pulverization of the obtained barium titanatepowder. The barium titanate powder can be easily pulverized by means ofdry pulverization using an atomizer, or jet mill or wet pulverizationusing a ball mill, planetary mill, or beads mill. Drying is performedusing oven, dryer, or spray dryer only when the wet pulverization iscarried.

The following examples are intended to further illustrate the presentinvention without limiting its scope. Further, the scope of the presentinvention is not limited to barium titanate powder but includespotential barium titanate-based powders depending on kinds and contentsof additives added.

EXAMPLE 1

1200 l of an aqueous 1 mol/l TiCl₄ and 1320 l of an aqueous 1 mol/lBaCl₂ were added to a glass-lined 4 M³ reactor and mixed. The mixturewas sprayed to 2520 l of an aqueous 1 mol/l oxalic acid in 6 M³ reactorthrough a single-fluid nozzle such as full-con at a rate of 21 l/min.The oxalic acid solution was stirred with a rate of 150 rpm and atemperature was maintained at 90° C. A diaphragm pump was used as asupplying pump for spraying the mixture solution. After spraying themixture to an oxalic acid for 2 hours, the mixture solution was stirredat a reaction temperature for 1 hour and at room temperature for 1 hourto produce barium titanyl oxalate slurry. The barium titanyl oxalateslurry was filtered using a centrifuge and washed with water till pH ofthe filtrate turned to above pH 6. Yield was 98% based on Ti ion and amole ratio of Ba to Ti was 0.999 (water content of 24%). 0.5 kg of 29wt. % aqueous ammonia (10.1 mol %, relative to the barium titanyloxalate) was added to 50 kg of the barium titanyl oxalate in 250 kg ofdeionized water. The slurry was stirred and the pH thereof before thepulverization was 9.3. Then, the slurry was performed for the wetpulverization to be 15 μm of maximum particle size with a beads mill.The pH of the slurry after the pulverization was 5.1. After the obtainedbarium titanyl oxalate slurry was dried in an oven at 120° C. for 12hours, the chloride ion content was 200 ppm. The barium titanyl oxalateslurry was then pyrolized at 1200° C. in an electric furnace andperformed for dry-pulverization to produce barium titanate powder. SEMimage of barium titanate powder was represented in FIG. 1.

EXAMPLES 2–6

Barium titanate powder was prepared by the same procedure as in Example1, except varying amount of aqueous ammonia during the wetpulverization. The characteristics of the obtained slurry weresummarized in Table 1.

COMPARATIVE EXAMPLE 1

Barium titanyl oxalate was prepared by the same procedure as in Example1 and the mole ratio of Ba/Ti was 0.999 (water content of 24%).

50 kg of the barium titanyl oxalate and 250 kg of deionized water wereplaced and stirred in the reactor. The pH of the slurry before thepulverization was 3.0. Then, the slurry was performed for the wetpulverization to be 15 μm of maximum particle size with a beads mill.The pH of the slurry after the pulverization was 2.0. After the obtainedbarium titanyl oxalate slurry was dried in an oven at 120° C. for 12hours, the chloride ion content was 10000 ppm. Then, barium titanatepowder was prepared by the same procedure as in Example 1.

TABLE 1 Before Used amount of pulverization After pulverization aqueousBa/Ti Content Ba/Ti Category ammonia (mol %) pH ratio* pH of Cl- ratio*Exam. 1 10.1 9.3 0.999 5.1 200 0.999 Exam. 2 0.4 8.3 0.999 3.0 500 0.999Exam. 3 0.5 9.0 0.999 5.0 200 0.999 Exam. 4 20 10.0 0.999 5.2 200 0.999Exam. 5 21 10.2 0.999 5.4 200 1.001 Exam. 6 25 10.2 0.999 5.4 200 1.005Com. — 3.0 0.999 2.0 10000 0.999 Exam. 1 Ba/Ti ratio*: XRF measurement

EXAMPLES 7–12

Barium titanate powder was prepared by the same procedure as in Example1, except using different additive with different amount during the wetpulverization. The characteristics of the obtained slurry weresummarized in Table 2.

TABLE 2 Before Additive pulverization After pulverization Cate- AmountBa/Ti Content Ba/Ti gory Additive (mol %) pH ratio* pH of Cl- ratio* Ex.7 Ammonium 10 6.8 0.999 5.1 200 0.999 acetate Ex. 8 Ammonium 8 5.5 0.9994.9 200 0.999 nitrate Ex. 9 triethylamine 15 10.2 0.999 5.0 200 0.999Ex. 10 Ammonium 14 7.9 0.999 4.8 200 0.999 carbonate Ex. 11 Ammonium 138.1 0.999 5.0 200 0.999 phosphate Ex. 12 arginine 12 8.0 0.999 4.8 2000.999 Ba/Ti ratio*: XRF measurement

EXAMPLE 13

Barium titanyl oxalate was prepared by the same procedure as in Example1, except addition of aqueous ammonia and additionally CaCO₃ and ZrO₂during the pulverization process. Then it was performed for dry,pyrolysis at 1190° C., and pulverization to produce(Ba_(0.952)Ca_(0.05))(Ti_(0.84)Zr_(0.16))O₃.

The obtained perovskite barium titanate powder has 0.52 μm of averageparticle size and 4.02 m²/g of specific surface.

COMPARATIVE EXAMPLE 2

(Ba_(0.952)Ca_(0.05))(Ti_(0.84)Zr_(0.16))O₃ was prepared by the sameprocedure as in Example 13, except that aqueous ammonia was not addedduring the pulverization process.

The obtained perovskite barium titanate powder has 0.54 μm of averageparticle size and 4.01 m²/g of specific surface.

EXPERIMENTAL EXAMPLE Determination of Conductivity Characteristics

A PVA binder and some additives for Y5V were added to each bariumtitanate powder prepared in Example 13 and Comparative Example 2. Eachmixture was well mixed in an alumina mortar and dried. Then, eachmixture was crushed in the mortar and sieved in order to obtaingranules. 0.4 g of each granule was pressed into a Φ 10 mm disc in amold. Then some dielectric characteristics were tested and the resultsare shown in Table 3.

TABLE 3 Green Sintering Die- Insulating Cate- density density Dielectriclectric resistance gory (g/cm³) (g/cm³) Constant loss (%) (10¹¹ Ω) TCC*(%) Ex. 3.69 5.90 12100 3.72 0.44 −60.3/10.2 13 Com. 3.68 5.89 100004.57 0.01 −67.2/13.8 Ex. 2 *TCC: Temperature coefficient of capacitance

As shown in Table 3, the barium titanate powder (Example 13) prepared byusing an additive during the wet pulverization process according to thepresent invention has superior conductivity characteristics (e.g.,higher dielectricity, higher insulating resistance, lower dielectricloss) compared to the barium titante powder (Comparative Example 2)prepared by the conventional method.

As described above, in the preparing process of barium titanate powderof high quality employing oxalate-derived method, the present inventionprovides an improved manufacturing process of the barium titanate powderby employing a nitrogen-containing additive such as amines to bariumtitanyl oxalate during the wet pulverization which prevents from theacidification of the slurry and decreasing the dielectriccharacteristics of the powder before and after the pulverization due tothe presence of chloride ions. It is, therefore, suitable for asmaterials for multilayer ceramic capacitors, PTC thermistors, resistors,and the like.

1. A method for preparing barium titanate powder comprising the stepsof: precipitating barium titanyl oxalate from a mixture of aqueousbarium chloride and titanium tetrachloride in an aqueous oxalic acid byspraying via a nozzle in high speed, aging, filtering and washing toobtain barium titanyl oxalate; wet pulverizing the obtained bariumtitanyl oxalate after adding a nitrogen-containing additive selectedfrom the group consisting of ammonia, amines, ammonium compound, andamino acids by means of a beads mill, drying and pyrolyzing to producebarium titanate powder; and re-pulverizing the obtained barium titanatepowder.
 2. A method for preparing barium titanate powder of claim 1,wherein said nitrogen-containing additive is used in the range of 0.5 to20 mol % relative to barium titanyl oxalate.
 3. A method for preparingbarium titanate powder of claim 1, wherein said amine is expressed bythe following formula (1),

wherein R₁, R₂, and R₃ represent individually hydrogen atom or C₁–C₅alkyl.
 4. A method for preparing barium titanate powder of claim 3,wherein said amine is selected from the group consisting of methylamine, dimethylamine, trimethylamine, diethylamine, and triethylamine.5. A method for preparing barium titanate powder of claim 1, whereinsaid ammonium compound is expressed by the following formula (2),

wherein R₁, R₂, and R₃ represent individually hydrogen atom or C₁–C₅alkyl; A represents hydroxyl (n=1), nitrate (n=1), sulfate (n=2),phosphate (n=3), carbonate (n=2), bicarbonate (n=1), C₁–C₁₀ carboxyl(n=1), or C₂–C₁₀ dicarboxyl group (n=2).
 6. A method for preparingbarium titanate powder of claim 5, wherein said ammonium compound isselected from the group consisting of ammonium hydroxide, ammoniumcarbonate, ammonium acetate, ammonium phosphate, ammonium oxalate,ammonium bicarbonate, and trimethylammonium hydroxide.
 7. A method forpreparing barium titanate powder of claim 1, wherein said amino acid isselected from the group consisting of arginine and lysine.
 8. A methodfor preparing barium titanate powder of claim 1, wherein a divalent ortetravalent metal is used with said nitrogen-containing additive.
 9. Amethod for preparing barium titanate powder of claim 8, wherein adivalent metal is at least on selected from the group consisting of Mg,Ca, Sr, and Pb.
 10. A method for preparing barium titanate powder powerof claim 9, wherein said additive is selected from the group consistingof oxides, carbonates, chlorides and nitrates of said metal.
 11. Amethod for preparing barium titanate powder of claim 8, wherein saidtetravalent metal is at least one selected from the group consisting ofZr, Hf, and Sn.
 12. A method for preparing barium titanate powder ofclaim 11, wherein said additive is selected from the group consisting ofoxides, carbonates, chlorides and nitrates of said metal.
 13. A methodfor preparing barium titanate powder of claim 8, wherein said additiveis selected from the group consisting of oxides, carbonates, chloridesand nitrates of said metal.
 14. A method for preparing barium titanateof claim 1, wherein the aqueous barium chloride has a concentration of0.2 to 2.0 mol/l.
 15. A method for preparing barium titanate of claim 1,wherein the titanium tetrachloride has an aqueous concentration of 0.2to 2.0 mol/l.
 16. A method for preparing barium titanate of claim 1,wherein the aqueous oxalic acid has a concentration of 0.2 to 5.0 mol/l.17. A method for preparing barium titanate of claim 1, wherein theprecipating step is effected at a temperature of from 20 to 100° C. 18.A barium titanate powder produced by the method of claim 1, having ahigher insulating resistance and lower dielectric loss as compared tobarium titanate powder produced by a wet pulverization process ofpreparing barium titanate without using said nitrogen-containingadditive.
 19. A barium titanate powder of claim 18, wherein saiddielectric constant is about 20% greater than that produced by said wetpulverization process without using said nitrogen-containing additiveand wherein said nitrogen-containing additive is ammonia.